1. Technical Field
The present invention relates to a polarization converting element, and a polarization converting unit and a projection-type imaging device each including the polarization converting element.
2. Related Art
A projection-type imaging device such as a liquid crystal projector is configured to modulate light emitted from a light source device in accordance with image information and enlarge and project the modulated optical image on a screen. In this liquid crystal projector, a polarization converting element is used in order to improve light utilization efficiency. The polarization converting element splits light having random polarization emitted from the light source device (hereinafter referred to as random light) into a plurality of intermediate light beams, converts the split intermediate light beams into one type of linearly polarized light beams, and outputs the linearly polarized light beams in a unified manner. The random light means light in which P-polarized light and S-polarized light of which the polarization planes are orthogonal to each other are mixed, or in which polarized light such as linearly polarized light, circularly polarized light, or elliptically polarized light of which the directions of the polarization planes are diversely mixed.
The polarization converting element has a structure in which a polarization separating film and a reflecting film are alternately disposed inside a transparent member to form a polarization beam splitter array, and a retardation plate is provided on a surface of the polarization beam splitter array. A plurality of retardation plates are disposed every predetermined interval on an exit-side surface of the transparent member at positions corresponding to the polarization separating films (see JP-A-2000-298212, for example).
In the related art, a ½-wavelength plate formed of an organic material such as, for example, a polycarbonate film is used as the retardation plate, and the ½-wavelength plate and the polarization beam splitter array are bonded by an organic adhesive agent.
The polarization converting element may be manufactured by the following method. A first translucent substrate formed of a colorless transparent glass, in which a polarization separating film and a reflecting film are formed on both principal surfaces thereof, respectively, and a second translucent substrate in which nothing is formed on both principal surfaces thereof are alternately stacked on each other to form a stacked structure. Alternatively, a first translucent substrate in which a polarization separating film is formed on one surface thereof and a second translucent substrate in which a reflecting film is formed on one surface thereof are alternately stacked on each other to form a stacked structure. The stacked structure is cut at an angle of 45 deg, for example, with respect to a plane parallel to the stacked surface to obtain a polarization beam splitter array. A ½-wavelength plate is bonded to an exit surface of the polarization beam splitter array by an adhesive agent.
The polarization converting element manufactured in this way is mounted in an optical engine of a liquid crystal projector in a state of being incorporated into a frame having a rectangular surface (see Japanese Patent No. 3610764, for example).
As the output power of a white light source lamp increases and the arc length thereof decreases, a thermal load imposed on a polarization beam splitter array and a ½-wavelength plate bonded to the polarization beam splitter array also increases. Thus, the use of a quartz crystal as a constituent material of the ½-wavelength plate is considered. A liquid crystal projector applicable to a case where a ½-wavelength plate is disposed to be bonded to the exit surface of the polarization beam splitter array by an adhesive agent is known. In the liquid crystal projector, an adhesive agent made of an ultraviolet curable resin or an inorganic material having excellent resistance to heat and light is used as the adhesive agent so that forced air cooling by a cooling fan becomes unnecessary (see JP-A-2009-103863, for example).
A polarization converting element in which a stacked structure obtained by repeatedly stacking a translucent plate member having two parallel surfaces, a reflecting film, a translucent plate member having two parallel surfaces, a ½-wavelength plate, and a polarization separating film in that order is cut at a predetermined angle with respect to the stacked surface thereof, so that the polarization separating film, the retardation plate, and the reflecting film are disposed in a state of being inclined in the same direction with respect to the cutting surface, and an incident surface and an exit surface are formed so as to be parallel to each other is known (see Japanese Patent No. 4080265, for example). A polarization converting element having the same structure as Japanese Patent No. 4080265 in which a Y-cut quartz crystal plate (the angle between a normal of a principal surface of a substrate and a crystal optical axis is 90 deg) having a thickness of 22.7 to 37.1 μm is disposed in an inclined state as the ½-wavelength plate is known (see JP-A-2009-128568, for example). A ½-wavelength plate made up of two wavelength plates in which first and second quartz crystal plates are stacked in a state where the plate thicknesses are set to 21.2 μm to 50.0 μm and 13.5 μm to 31.9 μm, respectively, and first and second optical axis azimuth angles of the optical axes thereof are 16.3 deg and 59.6 deg, respectively, is proposed (see JP-A-2009-244520, for example). In the related art of JP-A-2009-244520, polarization conversion efficiency (referred to as tricolor polarization conversion efficiency) of 0.8 or more is obtained when averaged by a tricolor wavelength region (wavelengths of 400 nm to 700 nm).
In recent years, as the demand for extending the service life of optical components increases, deterioration of an adhesive agent has become a problem.
To solve this problem, a method of bonding two translucent substrates formed of a glass or a quartz crystal is proposed (see Japanese Patent No. 4337935, for example). In this bonding method, a bonding film including an Si-skeleton having siloxane (Si—O) bonds on its surface and the degree of crystallinity of 45% or less and elimination groups including organic groups bonded to the Si-skeleton is formed by a plasma polymerization method. When energy is applied to the bonding film, the elimination groups present near the surface of the bonding film are eliminated from the Si-skeleton, so that the region of the surface of the bonding film develops a bonding property, whereby the two translucent substrates are bonded together.
By employing this bonding method, the bonding means is made inorganic, and the problem of deterioration of the bonding film is solved. Moreover, it is possible to achieve a long service life of optical components bonded using the bonding method.
In the related art, a polarization converting element which has an incidence surface and an exit surface substantially parallel to the incidence surface is proposed (see JP-A-2010-60770, for example). In the polarization converting element, a plurality of transparent members, a polarization separating film, a reflecting film, a phase plate, and a plasma polymerization film are disposed along the incidence surface and the exit surface. Either the polarization separating film or the reflecting film is provided on the inclined surface of some of the plurality of transparent members. The plasma polymerization film is provided at least one of the surface of the inclined surface of the transparent member, the surface of the polarization separating film, and the surface of the reflecting film. In the related art of JP-A-2010-60770, the plasma polymerization film achieves molecular bonding at least between the adjacent transparent member and reflecting film, the adjacent transparent member and phase plate, and the adjacent phase plate and polarization separating film, and the plasma polymerization film is mainly made up of polyorganosiloxane.
However, in the related art of JP-A-2010-60770, the plasma polymerization bonding film has a very small thickness in the order of tens of nm. When a foreign material such as dust adheres on the surface of the translucent substrate in the course of forming the bonding film on the surface of the translucent substrate using a plasma polymerization method, since the height of the foreign material is much larger than the thickness of the bonding film, the translucent substrates are not bonded together in a predetermined region around the region where the foreign material adheres. Thus, there is a problem in that bubbles or the like may be included in the region, which has an adverse effect on optical characteristics, bonding reliability, and product service life.
WO98-23993 is an example of a related art which does not use the plasma polymerization film. In WO98-23993, an optical block has a configuration in which optical components such as a planar polarization beam splitter (PBS), a mirror, and ½-wavelength plate are mounted on a groove formed on a substrate. The PBS is formed by depositing, on a surface of a glass plate, a dielectric multi-layer film or the like obtained by stacking alternately and repeatedly TiO2 (high refractive index material) and SiO2 (low refractive index material), for example. The PBS is press-fitted to the substrate at a predetermined angle with respect to an incidence direction of light. The mirror is formed by depositing aluminum, a dielectric multi-layer film, or the like, for example, on a surface of a rectangular glass plate, so that incident light can be reflected by the mirror. The mirror is mounted on the substrate at such an angle that an S-wave reflected by the PBS is reflected to an exit side. The ½-wavelength plate is formed by bonding a uniaxially stretched ½-phase difference film of polycarbonate, polyvinyl alcohol, or polyethylene terephthalate, for example, to a rectangular glass plate. The ½-wavelength plate is mounted at a position where the S-wave (S-polarized light beam) reflected by the mirror is incident, and the S-wave is converted into a P-wave (P-polarized light beam) and output. By forming the optical block using the PBS, the mirror, the ½-wavelength plate, and the like, it is possible to polarize randomly polarized incident light including P-wave (P-polarized light beam) and S-wave (S-polarized light beam) and output only P-wave (P-polarized light beam) in a unified manner. Moreover, it is possible to make the areas of the incidence and exit sides substantially identical to each other.
A quartz crystal has optically rotatory power as well as birefringent properties. It is well known that there is a problem in that the optically rotatory power has an influence on the retardation characteristics of a quartz crystal wavelength plate.
To solve this problem, a ¼-wavelength plate in which two wavelength plates formed of an optical crystal material having optically rotatory power are stacked so that the crystal optical axes thereof cross each other at a predetermined angle is proposed (see JP-A-2005-158121, for example). In the ¼-wavelength plate, the trajectories of polarized light beams are analyzed using the Poincare's sphere model, and the relations between a birefringent phase difference of both wavelength plates, an optical axis azimuth angle, optically rotatory power, and the angle between a rotation axis and a neutral axis are configured to satisfy a predetermined relational expression obtained by an approximate expression. By doing so, the influence of optically rotatory power is suppressed, and better wide-band characteristics are obtained.
A ¼-wavelength plate made up of one wavelength plate formed of an inorganic material such as a quartz crystal is proposed (see JP-A-2010-134414, for example). The ¼-wavelength plate is formed of a crystal plate formed of an inorganic material such as a quartz crystal which has birefringent properties and optically rotatory power and which exhibits reliability and sufficient resistance to blue-violet laser having a very short wavelength and high output power. The ¼-wavelength plate has excellent optical characteristics capable of obtaining optimum ellipticity of 0.9 or more or a value substantially close to 1.
Furthermore, a polarization converting element in which a ½-wavelength plate formed of a quartz crystal substrate is disposed to be inclined at 45 deg, a wire grid polarizer is disposed on an incidence surface side to function as a polarization beam splitter, and glass substrates having a reflecting mirror formed on the principal surfaces thereof are alternately disposed in parallel to the principal surface of the ½-wavelength plate is known (see JP-A-2004-029168, for example). A wire grid-type polarizer in which linear metallic thin wires are disposed at same intervals in parallel to each other on a transparent polycarbonate plate having birefringent properties or a substrate formed of an inorganic material such as calcite is known (see Japanese Patent No. 4527986, for example). In this wire grid-type polarizer, the direction where the refractive index is lowest within the substrate surface is orthogonal to the longitudinal direction of the metallic thin wires, and the substrate having birefringent properties is a ½-wavelength plate. Moreover, the direction where the refractive index is lowest within the substrate surface crosses the longitudinal direction of the metallic thin wires at an inclination angle of 45 deg.
However, in the related art disclosed in WO98-23993, the PBS is a polarization separating film which is formed by depositing, on the surface of the glass substrate, a dielectric multi-layer film or the like obtained by stacking alternately and repeatedly TiO2 (high refractive index material) and SiO2 (low refractive index material) on a glass plate. Therefore, there is a problem in that peeling may occur at the interface between the glass substrate and the polarization separating film due to thermal strain resulting from a difference in thermal expansion coefficient. Moreover, the glass plate has a limited heat-dissipation effect. Therefore, it is difficult to sufficiently meet the increasing demand for heat resistance and long service life.
Therefore, by taking a heat-dissipation effect into consideration, by forming the PBS on the surface of a quartz crystal plate instead of the glass plate as disclosed in JP-A-2004-029168, it is possible to realize a polarization converting element having heat resistance and long service life.
However, as described above, since a quartz crystal has optically rotatory power as well as birefringent properties, it is difficult to solve the problem of optically rotatory power just by using the quartz crystal plate instead of the glass plate and determining the azimuth of the crystal optical axis so as to create a phase difference (180 deg) from the relation with the polarization plane of incident linearly polarized light. Therefore, there is a problem in that an optical effect resulting from the optically rotatory power occurs in the incident linearly polarized light.
Therefore, the present inventors have applied the technique related to compensation of optically rotatory power as disclosed in the related art of JP-A-2005-158121 or JP-A-2010-134414 which focuses on the influence of the optically rotatory power on the phase difference. The present inventors have studied a polarization converting element capable of creating an optical effect such that the polarization plane of an incident P-polarized light beam is rotated by 90 deg to convert the P-polarized light beam into an S-polarized light beam and output the S-polarized light beam.
First, an application in which the technique proposed in JP-A-2005-158121 or JP-A-2010-134414 is applied to the optical design of a quartz crystal ½-wavelength plate which is disposed at an inclination angle of 45 deg so as to be inserted in the stacking interface of a prism array (a polarization beam splitter array) disclosed in JP-A-2009-128568 or JP-A-2009-244520 will be considered based on FIG. 20.
In this case, the transparent substrates interposing the quartz crystal ½-wavelength plate are formed of a general glass, and the refractive index n1 of the glass is 1.53, and the refractive index n2 of the quartz crystal is 1.54. Thus, light passes through the polarization converting element substantially with no change of the optical path (optical axis) of light passing through the prism array. That is, refraction occurs scarely at the interface between the glass and the quartz crystal ½-wavelength plate when light is incident to the quartz crystal ½-wavelength plate and at the interface between the quartz crystal ½-wavelength plate and the glass when light exits from the quartz crystal ½-wavelength plate.
In FIG. 20, when the optical axis azimuth seen from the normal PL of the principal surface (incidence or exit surface) of the quartz crystal ½-wavelength plate WP is θ0, and the optical axis azimuth with respect to the light beam R1 advancing in the quartz crystal ½-wavelength plate WP is θ1, and the angle between the normal PL of the principal surface of the quartz crystal ½-wavelength plate WP and the light beam R1 is θ2, these angles satisfy the following relation.θ0=a tan(tan θ1×cos θ2)  (A1)
In this case, since θ1=45 deg, and θ2=45 deg, θ0 is calculated as follows.
                              θ          0                =                ⁢                  atan          ⁡                      (                                          tan                ⁡                                  (                                      45                    ⁢                                                                                  ⁢                    deg                                    )                                            ×                              cos                ⁡                                  (                                      45                    ⁢                                                                                  ⁢                    deg                                    )                                                      )                                                  =                ⁢                  atan          ⁡                      (                          1              /                              2                                  1                  /                  2                                                      )                                                  =                ⁢                  35.3          ⁢                                          ⁢          deg                    
However, in the polarization converting element in which a quartz crystal ½-wavelength plate is disposed to be inclined at 45 deg, a polarization separating portion made up of a wire grid polarizer, a dielectric multi-layer film, and the like is disposed on the incidence surface side to function as a polarization beam splitter, and reflecting portions are alternately disposed in parallel to the ½-wavelength plate at an inclination angle of 45 deg as proposed in JP-A-2004-029168, the quartz crystal ½-wavelength plate is not interposed by glass plates but air is in contact with the quartz crystal ½-wavelength plate. That is, since the refractive index n0 of air is 1.00, and the refractive index n2 of quartz crystal is 1.54, the optical path (optical axis) of light passing through the polarization converting element is changed. This is because refraction occurs at the interface between the air and the quartz crystal ½-wavelength plate when light is incident to the quartz crystal ½-wavelength plate and the interface between the quartz crystal ½-wavelength plate and the air when light exits from the quartz crystal ½-wavelength plate.θ0=a tan(tan θ1×cos θ2)  (A1)
In this case, since θ1=45 deg, and θ2=27.2 deg, θ0 is calculated as follows.
                              θ          0                =                ⁢                  atan          ⁡                      (                                          tan                ⁡                                  (                                      45                    ⁢                                                                                  ⁢                    deg                                    )                                            ×                              cos                ⁡                                  (                                      27.2                    ⁢                                                                                  ⁢                    deg                                    )                                                      )                                                  =                ⁢                  atan          ⁡                      (                          1              ×              0.88941                        )                                                  =                ⁢                  41.65          ⁢                                          ⁢          deg                    
The optical design of the quartz crystal ½-wavelength plate when the quartz crystal ½-wavelength plate is inclined at 45 deg and is in contact with the air, so that the optical axis of incident light is refracted at the interface between the air and the incidence and exit surfaces of the quartz crystal ½-wavelength plate was examined. The examined design specifications are as follows.
Design wavelength: 520 nm
Design phase difference: 460.11 deg
Optical axis azimuth: 41.65 deg
Cutting angle: 90 deg
The cutting angle is defined as the angle between the crystal optical axis and the normal of the principal surface of the quartz crystal ½-wavelength plate. The design phase difference is defined as a phase difference when light having a design wavelength λ is incident from a direction parallel to the normal of the principal surface of the quartz crystal ½-wavelength plate. The optical axis azimuth angle (θ0) is defined as the angle between the crystal optical axis and the polarization plane of a linearly polarized light beam of the incident light as seen from the normal of the principal surface (incidence or exit surface) of the quartz crystal ½-wavelength plate. The relation between the wavelength of the quartz crystal ½-wavelength plate and the polarization conversion efficiency is illustrated as design values in the graph of FIG. 21.
When the distance of the optical path inside the quartz crystal ½-wavelength plate is t1, a phase difference Γ of the light passing through the quartz crystal ½-wavelength plate through the optical path is calculated by the following relational expression.Γ1=2π/λ×(ne−no)×t1
In this expression, the length t1 of the optical path is determined so that Γ1=180 deg. Moreover, the thickness “to” in the normal direction of the principal surface of the quartz crystal ½-wavelength plate is determined, and the phase difference Γo at the design wavelength λ in the normal direction is calculated.Γo=2π/λ×(ne−no)×to cos(θ2)=to/t1to=t1×cos(θ2)Γo=2π/λ×(ne−no)×t1×cos(θ2)
Γo is defined as a design phase difference, and in this example, Γo=160.11 (deg).
However, the quartz crystal ½-wavelength plate is manufactured by fragmenting a wafer obtained by cutting a quartz crystal Lambert obtained by shaping (Lambert processing) a quartz crystal ore at a predetermined cutting angle serving as a design value using a wire saw or the like.
However, in the manufacturing steps, when the wafer is cut from the quartz crystal Lambert at a cutting angle deviated from the design value or out of an allowable range, and the wafer is processed to the design plate thickness “to” described above, a deviation may occur in the design phase difference Γ0 as shown in Table 1. That is, Γo≠160.11 deg. Therefore, there is a problem in that all of the quartz crystal ½-wavelength plates become defective products.
TABLE 1Design valueAngular ErrorDesign wavelength (nm)520520Design phase difference (deg)160.11175.50Optical axis azimuth (deg)41.6541.65Cutting angle (deg)9080
When the cutting angle is deviated to 80 deg from the design value of 90 deg, if the wafer is processed to the design plate thickness “to” without taking the angular deviation into consideration, the design phase difference Γo will be greatly deviated to 175.50 deg from the design phase difference of 160.11 deg.
This is because the extraordinary refractive index ne and the ordinary refractive index no in the expression of Γo=2π/λ×(ne−no)× to depend on the cutting angle, and these values are changed with the cutting angle. Therefore, the phase difference Γ of light having passed the distance t1 of the optical path inside the quartz crystal ½-wavelength plate disposed at an inclination angle of 45 deg is greatly deviated from 180 deg.
As a result, the polarization conversion efficiency changes greatly as shown in FIG. 21. Therefore, there is a problem in that the polarization conversion efficiency at wavelengths of 550 nm or shorter decreases greatly and deteriorates.